1. Field of the Invention
The present invention relates to a control method for a movable swash plate in an automatic transmission mounted on a vehicle, especially a hydrostatic type continuously variable transmission wherein a swash plate type hydraulic pump of a fixed displacement and a swash plate type hydraulic motor of a variable displacement are connected together through a hydraulic closed circuit.
2. Description of Related Art
A hydrostatic type continuously variable transmission is known and is applied to transmissions of various vehicles, including two-wheeled motor vehicles. A control method for such a hydrostatic type continuously variable transmission is disclosed in Japanese Patent No. 2527199, in which an actual Ne (number of revolutions, also in the following) based on the Ne of a crank shaft for example is compared by a controller with a target Ne which is determined on the basis of preset conditions. Furthermore, the angle of inclination of a movable swash plate is adjusted to control the output.
As shown in FIG. 6 of the above-identified reference, in a continuously variable transmission, LOW and TOP positions in a change gear ratio are usually detected and the angle of inclination of a movable swash plate is usually controlled in the detected range.
According to the above related art control method, the change gear ratio in a hydrostatic type continuously variable transmission varies only in a predetermined range and the ordinary change gear ratio of TOP is 1.0. If the ratio is made smaller (OVERTOP), a lowering of efficiency will result. On the other hand, since a vehicular maximum speed depends on both the TOP ratio and the engine speed, a vehicle which is incapable of taking a large maximum value of engine speed becomes much lower in maximum speed. If the TOP ratio in a constant-speed vehicular traveling is set small in an effort to increase the maximum speed, the power performance will be deteriorated accordingly. Therefore, it is an object of the present invention to permit vehicular overtop traveling while suppressing the deterioration of power performance as far as possible in a continuously variable transmission.
According to the present invention, in order to solve the above-mentioned problems, there is provided a control method in a hydrostatic type continuously variable transmission, wherein a target engine speed of a target change gear ratio is determined on the basis of the degree of opening of a throttle valve and engine speed, and the change gear ratio of the continuously variable transmission is controlled on the basis of the determined value, characterized in that vehicular overtop traveling is allowed only when predetermined conditions are satisfied.
According to the present invention, in a control method for a hydrostatic type continuously variable transmission, since the change gear ratio of overtop is allowed only when predetermined conditions are satisfied, a constant speed traveling other than the overtop traveling does not deteriorate the power performance by setting the ratio at TOP. Besides, only under limited conditions, the maximum speed can be increased by overtop traveling. Thus, only when constant speed traveling is required which does not cause deterioration of power performance, it is possible to make the maintenance of power performance and the overtop traveling compatible with each other.
Furthermore, by setting the xe2x80x9cpredetermined conditionsxe2x80x9d referred to above as follows:
(1) The current change gear ratio should be at TOP position;
(2) The degree of opening of a throttle valve should exceed a threshold value determined according to a vehicle speed; and
(3) The engine speed should exceed a preset value, the conditions in question can be set strictly in only a vehicular traveling scene which really requires overtop permitting conditions.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.